a) Field of the Invention
This invention relates to the so-called "hollow double-structured pipe bending apparatus", in which a double-structured pipe for use in an exhaust pipe of an internal combustion engine of, for example, an automobile, and so forth is positioned substantially concentrically with a predetermined space gap being kept between an inner pipe and an outer pipe.
b) Description of Prior Arts
Various proposals have heretofore been made for such hollow double-structured pipe bending apparatus. As an example, there have been known those apparatuses for bending the double-structured pipe in a predetermined shape with a predetermined space gap being maintained between the inner pipe and the outer pipe, wherein, for example, an inner core metal is inserted into the inner pipe and a tubular core metal is interposed in a space between the inner pipe and the outer pipe, and one end of each of the inner pipe and the outer pipe is fitted and held on a bending dies, followed by rotating the die to bend both inner pipe and outer pipe simultaneously along the shaping face of the bending die (vide, for example, Japanese Patent Publication No. 55-24971).
With the conventional pipe bending apparatus, however, such hollow double-structured pipe could not necessarily be bent as desired, with the consequence that there have often accrued various inconveniences such that creases, etc. often took place on the inner peripheral side of the pipe in its bending direction (above all, on the inner peripheral side of the inner pipe), or the pipe itself was subjected to buckling, and others. In particular, with the exhaust pipe for those advanced modern internal combustion engines, demand has been raised in making the wall thickness of the inner pipe to be the thinnest possible for improving its cleaning efficiency of exhaust gas, or for attaining its light-weight. However, such light-weight pipe with its thin wall thickness tends to readily bring about the abovementioned creases and buckling.
The present invention is proposed in view of the abovementioned points of problem inherent in the conventional pipe bending apparatus, and aims at providing an improved hollow double-structured pipe bending apparatus which is capable of effecting good bending work on a double-structured pipe, even when an inner pipe of a thin wall thickness is used.